1. Field of the Invention
The present invention relates to an information recording system having a high recording density, and more particularly to a thermally assisted magnetic recording system including a probe of near-field light that irradiates a recording medium with light, and a magnetic read-write head.
2. Description of the Related Art
A magnetic disc system mounted on a computer or the like as one of information storage systems that support present-day information society is rapidly becoming higher in recording density, higher in speed, and smaller in size. In order for the magnetic disc system to achieve a high recording density, it is necessary to reduce a distance between a magnetic disc and a magnetic head, to make fine a crystal grain size that forms a magnetic layer of a magnetic recording medium, to increase the coercivity (or magnetic anisotropy field) of the magnetic recording medium, to speed up a signal processing method, and to do the like.
In the case of the magnetic recording medium, making the crystal grain size fine leads to a reduction in noise but causes the problem of rendering grains thermally unstable. KuV/kT is an index of thermal stability, where Ku denotes a magnetic anisotropy constant; V, the volume of the grains; k, a Boltzmann constant; and T, a temperature. The larger value of KuV/kT indicates higher thermal stability. The magnetic anisotropy constant must be increased in order to ensure the thermal stability, while making the crystal grain size fine. However, an increase in the magnetic anisotropy constant, that is, an increase in the magnetic anisotropy field (or the coercivity), means an increase in head-field intensity required for recording. It is considered to be difficult, from now, to increase the magnetic anisotropy field with increasing recording density, because of restrictions on the use of a material for a magnetic pole for a write head and restrictions on the reduction of the distance between the magnetic disc and the magnetic head.
Hybrid recording technique having a combination of optical recording technique and magnetic recording technique is proposed in order to solve the foregoing problems. Attention is being given to the hybrid recording technique. A read-write head described in Jpn. J. Appl. Phys. Vol. 38 (1999), pp. 1839-1840, for example, is provided with the addition of a mechanism that irradiates light in an area where a recording magnetic field is applied. During recording, the magnetic field is applied and the light is irradiated simultaneously. Thereby, the recording is performed by utilizing the effect of reducing the magnetic anisotropy field (or the coercivity) of the medium by heating via the light irradiation. In other words, the medium is heated via the light irradiation to thereby reduce the magnetic anisotropy field of the medium having a high magnetic anisotropy field at room temperature for achieving an ultrahigh recording density, on which a conventional magnetic head has difficulty in recording because of an insufficient recording magnetic field. Thereby, the read-write head facilitates recording. This recording method is called a “thermally assisted magnetic recording method.”
A thermally assisted magnetic recording system is basically considered as an extension of a conventional perpendicular recording system. A magnetic write head uses a single pole type head, and a read head uses an MR (magnetoresistive) head for use in conventional magnetic recording. However, the examination of the optimum configuration of the magnetic head and the examination of technique for achieving a merger between the magnetic head and the heating mechanism are important subjects for thermally assisted magnetic recording. A heat generating device has to rapidly heat and cool a tiny heating region to achieve a high recording density for the magnetic disc system. Accordingly, there is a limit to the approach of focusing laser light through a lens generally used for optical recording. The approach of generating near-field light by a metallic surface plasmon is proposed as an approach for solving this, and studies are carried out. See Japanese Patent Application Laid-open Publication No. 2003-45004, etc.
Recording methods close to the thermally assisted magnetic recording method include an optical magnetic recording method. In the optical magnetic recording method, a change in magnetic properties based on a temperature increase in the medium by laser light irradiation is utilized for magnetic recording. The recording methods are of some types. One of the types is the recording method that involves heating, to Curie temperature, a medium using TbFe (a terbium-iron alloy), GdTbFe (a gadolinium-terbium-iron alloy), or the like. Specifically, spontaneous magnetization decreases sharply in the vicinity of the Curie temperature, and paramagnetism develops at or above the Curie point. At this time, a magnetic field is applied in an opposite direction. In a cooling process, magnetization rotation occurs, so that a mark is recorded. Another type is the recording method that involves heating a medium made of GdFeO (a gadolinium-iron-oxygen alloy) or GdCo (a gadolinium-cobalt alloy) at or above a compensation point. This method utilizes a phenomenon given below. When two sublattice magnetizations of ferrimagnetism compensate each other at a temperature on which these magnetizations are dependent, macroscopic magnetization becomes zero (which is called a “compensation point”), so that coercivity is maximized. When such a material as has a compensation point at room temperature is heated at or above the compensation point, therefore, the coercivity is reduced, so that magnetization is oriented in the direction of an external magnetic field. The medium is made of an amorphous alloy film of rare earth and transition metal. A record mark is determined by forming a cylindrical magnetic domain. Formation of the magnetic domain is determined by a balance between some forms of magnetic energy (such as external magnetization energy and magnetic domain wall energy) acting on the medium. Because of having no grain boundary, the amorphous alloy film has the merit of achieving a low noise level as compared to a CoCr-base granular medium that has been heretofore used for the magnetic disc. For the formation of the magnetic domain, however, it is difficult to control the magnetic energy. It is particularly difficult to control magnetic domain wall motion by the action of the magnetic domain wall energy or to control the thickness of the magnetic domain wall. As the recording density becomes higher, the record mark may possibly become larger than the spot size of light or become rather smaller and disappear. Consequently, the amorphous alloy film is considered to be unsuitable for high recording densities.
The CoCr-base medium in current use as a perpendicular recording medium is the medium formed of fine crystal grains. Each individual grain is surrounded by an SiO2 (silicon oxide) layer that is a non-magnetic layer. The CoCr-base medium is known as the medium in which a considerably weak magnetic exchange interaction takes place between the grains. The medium in which each individual grain is surrounded by the non-magnetic layer as mentioned above is generally called a “granular medium.” In the granular medium, the magnetizations of the individual grains rotate independently, because a considerably weak magnetic exchange interaction occurs between the grains. The granular medium as applied to the thermally assisted magnetic recording method can achieve a desired uniform record mark, because of easily controlling the size of the record mark (or record bit) by using the magnitude of the magnetic field, the spot size of heat, the magnetic properties, or the like. In other words, the granular medium has a structure suitable for high recording densities and is very likely to be a medium for thermally assisted magnetic recording.
The thermally assisted magnetic recording is characterized by facilitating recording because the magnetic anisotropy field intensity (or the coercivity) of the medium is reduced by heating the medium, as mentioned above. In other words, the magnetic anisotropy field intensity of the medium has dependence on temperature, and therefore the magnetic anisotropy field intensity becomes lower as the temperature of the medium becomes higher. See J. Appl. Phs. 91, (2002) pp. 6595-6600. The head-field intensity required to perform saturation recording on the medium is known to be equal to or more than the magnetic anisotropy field intensity of the medium at the center of a magnetic recording layer in the direction of the thickness thereof. See Jpn. J. Appl. Phys. Vol. 43 (2004) pp. 6052-6055. In other words, when the head-field having the same intensity as the magnetic anisotropy field intensity is applied to the medium, magnetization rotates completely, so that recorded magnetization is saturated. For example, when a head-field intensity of 1000×103 A/m is applied to the medium having a magnetic anisotropy field of 1000×103 A/m, saturation recording can be performed. It is therefore possible that if light irradiation of the medium enables a reduction in the magnetic anisotropy field, the head-field intensity required for saturation recording can be proportionally reduced. For example, it can be easily expected that if the magnetic anisotropy field can be reduced to 200×103 A/m, a head-field intensity of 200×103 A/m is sufficient. Actually, the optical magnetic recording method close to the thermally assisted magnetic recording method achieves recording at a magnetic field intensity of 80×103 A/m or less by heating, to the vicinity of the Curie point, a medium having a magnetic anisotropy field of about 1600×103 A/m at room temperature. For this reason, even the thermally assisted magnetic recording method is expected to enable recording with a low magnetic field. See The Magnetics Society of Japan, Bulletin of topical symposium, No. 128 (2003) pp. 39-50. Furthermore, the results of computer simulation of thermally assisted magnetic recording are given in J. Phys.: Condens. Matter 17 (2005) R315-332. Specifically, the simulation involves irradiating a medium having a magnetic anisotropy field Hk of 860×103 A/m with light having a spot size of 150×200 nm, and applying a magnetic field at a low head-field intensity of 240×103 A/m, thereby performing recording at a recording density of about 100 Gbpsi. This simulation shows that a much lower magnetic field than Hk can be used for recording at an assumed recording density for thermally assisted magnetic recording, although the spot size is very large and both the recording density and Hk are excessively low.
The relation between head-field intensity and sidetrack erase for thermally assisted magnetic recording has been reported in MORIS 2006 WORKSHOP, Technical digest, TuA-02. An analysis method is an analysis of static behavior, not allowing for the speed of magnetization rotation. The relation between magnetic field intensity and magnetization rotation has been determined under the following conditions: the ratio between static magnetic energy (2μ0HMsV, where μ0 denotes the permeability of vacuum; H, an applied magnetic field; Ms, a saturation magnetic flux density; and V, the volume of grains) required for magnetization to rotate in the direction of desired head-field intensity in the vicinity of the Curie point and thermal fluctuation energy kT (where k denotes a Boltzmann constant; and T, an absolute temperature) exceeds 4, and the ratio between energy that rotates magnetization and thermally stabilizes recording magnetization (namely, magnetic anisotropy energy: KuV×(1−H/Hk), where Ku denotes a magnetic anisotropy constant) and thermal fluctuation energy kT exceeds 3. This report has shown that a magnetic field intensity of 480×103 A/m or more is necessary in order that magnetization undergoes 95% or more rotation.